1. Field of the Invention
The present invention relates to an electric power steering apparatus, more particularly, to a shape of a worm tooth portion in a worm wheel mechanism constructing a power assist section used in such an electric power steering apparatus.
2. Description of the Related Art
Generally, in an electric power steering apparatus having a power assist section driven by an electric motor, there is used a worm wheel mechanism as disclosed in Japanese Patent Publication (Kokai) No. 9-221045 (published on Aug. 26, 1997). With the adoption of such a worm wheel mechanism, an amplified assist power from the electric motor by the worm wheel mechanism is transmitted to a power steering apparatus. On the other hand, a load reaction force (kickback force) is not transmissively returned to a steering wheel through the worm wheel mechanism.
In the worm wheel mechanism of such an electric power steering apparatus thus-constructed above, it is required such that a backlash between a worm shaft and a wheel gear is restrained to be under a predetermined value, in order to obtain a predetermined feeling of a steering operation during a power assist operation in such an electric power steering apparatus. To reduce this backlash, it however is necessary to machine in high accuracy each of tooth portions and to maintain an axial distance between the worm shaft and the wheel gear accurately. As a result, this causes to increase a manufacturing cost.
Accordingly, an object of the present invention, to solve the above mentioned problem, is to provide an electric power steering apparatus that is capable of smoothly transmitting a torque through a worm wheel mechanism in which a compound lead is adopted on a worm tooth portion to reduce a backlash therein during a rotational operation of a worm wheel.
Briefly, an electric power steering apparatus according to the present invention is composed of a wheel gear connected with a steering wheel, a worm shaft tooth-engaged with the wheel gear, and an electric motor rotatably driving the worm shaft in which a worm tooth portion of the worm shaft is formed with a compound lead. Further, the worm shaft is fixed in an axial direction by at least one locknut provided at a portion of a separate side of the electric motor.
With this configuration, before the worm shaft is fixed in the axial direction by the locknut, the worm shaft is moved in the axial direction by an extremely small distance so as to optimize a backlash between the worm tooth portion of the worm shaft and the wheel gear. Thus, the worm shaft is moved only in the axial direction, so that an amount of the backlash can be adjusted to an optimized value without changing of an axial distance between the worm shaft and the wheel gear. Thereafter, the worm shaft is fixed in the axial direction by the locknut under condition that the backlash is maintained in the optimized value. Besides, such a tooth-engagement of the worm shaft with the wheel gear can be established on an entire tooth portion of the wheel gear, so that a torque transmission can be smoothly performed from the electric motor through the worm shaft to the wheel gear.
Further, one end of the worm shaft is rotatably supported through a ball bearing and is fixed by the locknut in the axial direction of the worm shaft, and the other end thereof is rotatably supported through another ball bearing and is fixed by a circlip through a disc-shaped spring in the axial direction.
According to this configuration, the worm shaft is positioned in the axial direction thereof under condition that the backlash is maintained in the optimized value. In such a situation, the worm shaft is urged by the disc-shaped spring and therefore, it is fixed under a condition that an axial force of the worm shaft is balanced. As a result, a position of the worm shaft can be maintained to be an optimized value of the backlash close to zero.
In the configuration described above, only respective outer rings of the ball bearings are further secured to a housing in which the worm shaft and the wheel gear are disposed. With this configuration, an attachment span between the both ball bearings can be diminished to be a distance close to a length of the worm tooth portion.
Further, a gear tooth portion of the wheel gear is formed with an ordinary lead that is not the compound lead. In the configuration described above, the machining on the gear tooth of the wheel gear can be facilitated.